Japanese Patent Application No. H11-160759 discloses a parallel type hybrid system which comprises: an engine, a transmission device for gearing the speed of rotation of an input shaft and transmitting said rotation to vehicle wheels via an output shaft, a clutch for engaging and disengaging the output shaft of the engine and the input shaft of the transmission device, a rotational electric device serving as an electric motor and an electric generator, a power transmission mechanism for coupling the input and output shafts of the rotational electric device with the input shaft of the transmission device, and an accumulator element for accumulating electrical power supplied by the rotational electric device.
In prior applications of this kind, if the amount of electrical charge accumulated in the accumulator element is sufficient, then the clutch is disengaged, and the vehicle is caused to accelerate or travel under the driving force of the rotational electric device. If the electrical power is consumed by driving operation of the rotational electric device and the accumulated charge in the accumulator element declines, then it is no longer possible to obtain an output corresponding to the required driving force (amount of operation of the accelerator) for the vehicle, from the rotational electric device.
Therefore, if the accumulated charge in the accumulator element falls to or below a prescribed value, the clutch is engaged and the traveling state of the vehicle is continued by means of driving force from the engine. In this case, the driving operation of the rotational electric device is limited to accelerating or low speed running, and hence the operational range of the engine becomes large, and sufficient improvement in fuel consumption or reduction in exhaust gases cannot be achieved.